BACKGROUND Despite advances in pain research, many patients still experience moderate to severe post-operative pain that needs to be alleviated. The primary aim of this study was to identify predictive factors for severe post-operative pain after thoracotomy. Secondary, we investigated the levels of post-operative pain in this group of patients. METHODS A prospective longitudinal study with 144 patients scheduled for thoracotomy was conducted between December 2007 and August 2010. All patients were given a questionnaire consisting of three sections (Brief Pain Inventory, Life Orientation Test-revised and Hospital Anxiety and Depression Scale) the day before surgery. Only those patients with pre-existing chronic pain were asked to complete all three sections. Post-operative pain scores were recorded three times a day for 6 days using the Numeric Rating Scale, and all analgesic consumption was recorded daily. RESULTS Fifty-four patients reported pre-existing pain before surgery, most commonly from the neck, shoulders, lower back, hips or knees. For the first 3 days after surgery, the pain scores for those who had pre-existing chronic pain and those who did not, where almost similar with no statistically significant difference. The total amount of the epidural analgesia-mixture used where also almost similar for both groups with no statistically significant difference. CONCLUSIONS In contrast to previous studies, which have reported pre-operative chronic pain to be a strong predictor of moderate to severe post-operative pain, we have in our study not been able to replicate these findings.